The .usr file is part of the UFT script's settings and configuration parameters. It is used in conjunction with the mdrv.exe file to actually perform the script replay. The mdrv.exe is the Main Driver application that loads the .usr file. A UFT script is actually a lot more complicated than other applications such as Microsoft Word or Microsoft Excel, in which you have a single file. A UFT script is actually stored in a folder with the test name and has many necessary files that are used for the execution of a test. At the following link, you can see an explanation of the Silent Runner that explains the mdrv.exe file in conjuction with a .usr file:

http://cc.bingj.com/cache.aspx?q=what+is+a+.usr+file+in+uft&d=4776636913683016&mkt=en-US&setlang=en-US&w=mHSOGPBrVl58Kc3FLz_4oSwYWxF_oEPH